The invention concerns a method for the manufacture of an electrode for electrochemical cells with a Raney rare-metal catalytic layer disposed on a metallic support structure in layer form and the electrodes made thereby.
Electrodes for electrochemical reactions with rare-metal catalysts, e.g., platinum black or Raney platinum, have a high activity. The rare-metals are usually also termed as noble metals. For preparing such electrodes, the active metal can be precipitated from a solution on a sheet of metal, e.g., in the manufacture of a platinized platinum electrode. However, the active metal can also be bonded to a contact screen, say, of tantalum or platinum, by a binder, such as, polytetrafluoroethylene Raney platinum, which is prepared from a platinum-aluminum alloy, can be processed into electrodes by means of pressing with gold powder or by means of a binder. Another method for the manufacture of electrodes for electrochemical cells is also known, in which a Raney rare-metal catalytic layer is prepared on a metallic support structure in layer form wherein the inactive alloy component are specifically aluminum and tin (see German Auslegeschrift No. 1,172,650).
Although such electrodes have proven themselves useful in conventional fuel cells, they are relatively thick, at least several tenths of a millimeter, and are not particularly useful in various applications, particularly in biofuel cells, where it is necessary to use thinner electrodes, i.e., electrodes with a thickness of about 0.1 mm or less. Such electrodes, however, cannot be made by known methods since the alloys used heretofore for the manufacture of the Raney metals have unfavorable mechanical properties, particularly they are very brittle. They can therefore not be processed into thin layers. Thus, the alloy PtAl.sub.4, for instance, which is most used, exhibits a low mechanical load capacity and it breaks very quickly in bending tests.
For thin and small electrodes it is furthermore necessary that they exhibit increased activity, so that a large output can be generated in a volume as small as possible. In the electrodes prepared by known methods, however, the content of binder has a detrimental effect. The activity basically available can be only partially utilized. On the other hand, electrodes pressed with gold are relatively heavy and are therefore not advantageous as far as the output per unit weight is concerned. Electrodes, which are not bonded either by gold or by other binders, lack the necessary mechanical stability. It has been found that known electrodes must further be improved with respect to efficiency and activity in order to be usable in biofuel cells.